Automatic gauge control



T. H. BLOODWORTH AUTOMATIC GAUGE CONTROL Filed Oct. 24. 1960 Feb. 16, 1965 United States Patent iltice lddl carentes een. is, rees fr i File-d Get. 24, Edil, Sver. No, 64:52@

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This invention relates to an automatic gauge control for a rolling mill. The control is particularly intended for a hot strip rolling mill.

The problem of obtaining accurate gauge control ma rolling mill has become more and more dihicult as mill speeds increase. At the same time customers have demanded that metals have more uniform gauge. rPhe gauge of the product of a rolling mill depends in part on the tension on the material in the rnill and on the pressure of the work rolls on the material. la addition, the hardness and the thickness of the material entering the mill iuiiuence the gauge. Automatic gauge controls have been suggested for hot strip mills in order to improve the uniformity of the metal as it enters the cold mill. An automatic gauge control for a hot rnill presents problems that do not exist in the automatic gauge controls for cold mills. in a hot mill, water cooled metal sl'ids which support the metal while it is being heated prevent the metal from heating uniformly. Also, the strip cools somewhat before it enters the mill, and the last portions to enter the mill are apprecinbly cooler and harder than the iirst portions. These and other variations in temperature and thickness may cause the mill to roll the strip ofi gauge.

The prior art has suggested automatically varying the roll pressure to produce the etect to a stiff mill. In a stift mill the spacing between the work rolls does not change even though the pressure on the work rolls changes .as the temperature or the thickness ofthe strip entering the mill changes. Analyzing some of the factors that inhuence gauge suggests that each stand of a sti mill would roll uniform gauge and that the interstand tensions would remain constant. However, many of the factors that inluence gauge have not yet been systematically analyzed, and the concept of a stift mill is not a completely satisfactory basis for an automatic gauge control.

It would be highly desirable to maintain constant interstand tension. Changes in strip tension tend to transmit the effects of a chanac in roll pressure at one reducing stand to the other stands. lf the interstand tension is nearly constant, the strip can be rolled at a lower tension than if the iuterstand tension varies widely. However, controls of the known prior art have not maintained constant tension but usually have varied the interstand tension to regulate the strip thickness. Other prior art mills have varied the work roll pressure only in a narrow range and in this way have kept the tension variations within an acceptable range.

The automatic gauge control of this invention senses the interstand strip tension and the error in strip thickness. When the material is ott` gauge, the control changes the roll pressure to correct the gauge. The control varies the speeds of the work rolls to maintain constant interstand tensions. The constant tension regulators isolate the eiiects of work roll changes at each stand. Under most rolling conditions the constant tension control ypermits the work roll pressure to be varied over a very wide range. However, under other conditions, wide variations in work roll pressure may increase the strip tension morethan the drive motors can compensate for. The strip tension would then Vary with roll pressure just as in a mill without this control. Excessive tension is highly undesirable because it tends to reduce the strip width as well as the strip thickness. In addition, excessive tension may cause the work rolls to slip, or it may break the strip. The tension sensing components of this control prevent the roll pressure from increasing or decreasing if the mill stand drive motors would be unable to oiset the resulting change in tension. The roll pressure limit feature of the control further increases the range that the work roll pressure can be varied under most conditions, and it thereby simplilies adapting the control to a number oi stands.

One object of this invention is to provide a new and improved automatic gauge control.

Another object of this invention is to provide a new and improved automatic gauge control that is particularly suited for a hot strip mill.

Another object of this invention is to provide a new and improved automatic gauge control that maintains constant interstand tension and varies the work roll pressure over a wide range.

Another object of this invention is to provide a new and improved automatic gauge control in which work roll pressure controls are used on several reducing stands independently, and interstand tension regulators isolate the effects of changes in roll pressureand roll spacing at each stand.

Other objects and advantages will appear from the drawing and from the following description of the invention.

in the drawing, FlG. l schematically illustrates an ernbodiment of the control of this invention in combination with a hot strip mill. Solid lined arrows indicate the low of information and orders between elements ot the control and the mill.

FIG. 2 schematically illustrates one of the components ofthe control of FIG. l.

FIG. 1 shows the automatic gauge control of this invention with a well known five stand hot strip mill. Elements or the mill and the control that are duplicated at several stands have a common number and in some cases are distinguished by letter suilixes. Each stand l, 2, 3, 4 and 5 has a pair of work rolls il and l2 and a pair of backup rolls le and i5 that a strip of metal lo passes between. A screw device 1S and a cooperating screw motor )i9 force the work rolls together against the strip, and direct current motors 21 rotate the work rolls through drive shafts 23 to pull the strip through the mill. The screw motors 19' are each energized and deenergized by an electrical controller 24 that receives a signal from the automatic gauge control. Each controller 2li may comprise two magnetically operated switches that connect the motor 19 to a source of electricalV power to run the motor in either direction to raise or to lower the screw le. The coils of the operating magnets of the switches receive the signal from the programmer 63. As each stand reduces the strip thickness, it increases the strip speed. The drive motors 21, except the drive mot-or 2lb ofthe second stand, have speed controls 2S that operate on the motor field windings 29 in response to speed control orders `from the automatic gauge control. The second stand is the pacer stand, and its motor 2lb is set to the desired speed by suitable means that is illustrated schen'laticall; by a held control rheostat Sill The motors Zia, c, 5J, e, of the other mill stands are setup to run at diering speeds that correspond to the speed of the motor 2lb of the second stand and the reduction in strip thickness between stand two and the other stands. The control varies the motor speeds as necessary to maintain constant interstand tension. Each speed control means comprises a magnetic amplifier 3l that receives and ainpliries an inupt signal from the control and an exciter generator 32; that further amplies the signal and energizesthe held winding 29 of the motor 21.

y i 3 1 torque to the looper support through aA shaft 51 and tends to raise the strip where it passes over the roller. When the interstand tension decreases, the looper raises the strip further and tends to prevent slack in the strip. When the interstand .tension increases, the strip forces the looper 5 down and lessens the tension increase.

The speciiic features of the millof the drawing illustrate generalfeatures that are found in many mills that Y might use this control. When vthe control itself-is understood, it will be apparent that various well known mill lo components may be substituted for the components that Y gare illustrated.

The automatic gauge control canbe understood more V veasily by first considering the operation of the mill withouta control. When the mill isset up for the desired gaugethe'work rolls 11, 12 of each stand are properly spaced apart to obtainthe desired 'reduction for theV expected .thickness and hardness of the entering strip 16,

' and Vthe vspeedsof the work roll drive motors 21 are setV to correspond to the desired reduction. So long as the 2O strip is the proper hardness. and thickness, the mill produces nearly uniform gauge. However,V when a harder rportion-of the strip enters the r'st stand, the work rolls spring apart slightly. Consequently, the exit gauge increases, the Vstrip exit speed decreases in proportion to the increase in gauge, and the tension increases between stands one and two. When the tension increases between stands V 1 and 2, but before the harder portion of the strip reaches Y the second stand, the second stand rolls the strip thinner than is desired. VWhen the harder portion of the strip enters the second stand, a similar effect occurs'between Vstands 2 and 3. In addition, the striptension increases .between stands 1 and 2 and causes the first stand to roll the strip thinner than desired. When the strip enters the iirst stand too thick, the rst stand rolls the strip thicker than desired. In addition, the strip tension between stands `one and two decreases and the second stand tends to roll the strip too thick even before the thicker portion of the Vstrip enters the-second stand. In automatic gauge conj trols of the prior art, changes in strip tension tendto trans- Q mit the eifects of roll pressure adjustments from stand to rstand. The changes in strip tension have limited the range of roll pressure adjustmentsin these priorv art/icontrols. Y

The automatic gauge control of this invention operates on the motor speed controls and on the screws in response to changes in strip thickness and changes in interstand tension. There are several well known devices thatmeai i sure tension and thickness. The drawing illustrates X-ray gauges 59. Y Y

One thickness gauge 52 is located at the exit side of the irst stand; and the other thickness gauge 53 is located` at j the exit side of the last stand. The gauges52, 53 may be *located at other stands it desired solong as each gauge 55 measures the product of a group of one or more of the mill stands. The gauges 52, 53 each comprise a radiation r source 55 that is positioned on one side of the strip- 16 and ff,

- a radiation receiver 56 that is positioned opposite the thickness gauges 52, 53 and looper following tension 50 'sorbsthe radiation in proportion to the strip'thickness, and the receivers 56 produce electricalsignals 57, S8 that indicate strip thickness as a' function of the proportion of/ v the radiation that penetrates the strip.; i v j The position of a looper 41 indicates the interstand ten- 65 sion. Asthe roller 47 and the strip 16 rise, the forcel Vof the strip on the roller increases vand the force of the looper motor 50 on the roller decreases trigonometrically. Similarly, as the roller andthe strip fall,'the downward force of the stripfdecreases andthe'upward force of the looper'v 70 motor increases trigonometrically. Thus, there is Vai de' terminableistable position of the looper for every value of strip tension.V A magnetic reluctance device 59 is coupled to the looper by a shaft 60 to follow 4the looper position.

:the motor 21a when the tension Vis low.

61 anda tension limit signal 62. The reluctance device may comprise a magnetic core that has a stationary niember with an air gap and a movable member that varies the length of the air gap. The movable member, which is connected to the shaft 60, changes the reluctance of the core as the looper 41 turns the shaft et). Coils on the Vcore produce an electrical signal that varies with the reluctance of the core, and thus with the tension of the strip 16. Y

Each reluctance device 59 feeds a tension error signal Y 61 into the amplifier 31 of its associated mill stand motor speed control 2S. The speed control 2S adjusts the work roll speed in response to tensionsignal 61 to maintain the proper strip tension., The exciter 28a of the iirst mill stand motor speeds up the motor 21a when the tension is high between stands 1 and the pacer stand 2, and it slows VThe millstand motors 21C, d and e of stands 3, t and 5 have the opposite speed-tension relationship to the pacer stand 2, and the controls of these motors respond to tension errors in the reverse sense to the tirst stand 1. For example, when the tension rises between stands 2 and 3, the looper follower 59c orders the motor 21C to slow.

When the thickness gauge 52 senses a thickness error, it is desirable to operate the screw motor 19a of stand 1 .for an increment that is' proportional to the thickness error and then to deenergize the screw motor long enough for the corrected portion of the strip 16 to reach the thickness gaugeV 52. The control then orders an additional ychange in the roll pressure if this is necessary. The control for the first stand includes a programmer 63a, shown in detail in FIG. 2, that combines the thickness error sigf'nal 57 with other information and orders appropriate changes'in the work roll pressure.

cuit that is suitable for programming the incremental Vchanges compares an error with the rate that the system is changing in order to correct the error.' The screws 18 have suitable means that provide an electrical signal that indicates the screw position. Y For example, the position indicator-may comprise a synchro transmitter mechanically'rconnected to rotate with the screw 1 8 and electrically connected to drive a synchro receiver in the programmer 65, and the synchro receiver may convert the screw rotation information to Van analogous electrical quantity by positioning the slider of apotentiometer. The screws 18 feed back to the programmer a screw position Y signal 65 which indicates the rate that the mill stand is responding to the control. When the rate of'correctionof the screw position signal 65 reaches a predetermined balance with the thickness error signal 57, the circuit orders the controller 24 to deenergize the screw motor 19a.

In FIGI. l2 the means for energizing the controller 2li in response-to a balance between a thickness error and the Y rate oroperating to correct the error' is illustrated as an adder 656 that i'sconnected to comparethickness error signal 57and a rate signal 67 and 'toV produce an output that is afunction of their difference. Rate signal 67 is derived vfrom screw position signal 65a by means 68 such* as a dierentiatingdevice illustrated schematicallyby a casource'on therother side of the strip. The strip 16 ab- 60( the correctedl portion of the strip to reach the thickness f gauge 52'.tv Strip" speed indicators'tl are coupled to the Vstrip 16` through thedriveshafts 2.3 or other suitable The strip speed indicator tproduces a strip means. speed signal 72 that is fed into the programmer 63. After l t each incremental voperation o' the screws 13a, the programmer 63a orders the controllerzda to deenergizeV the y Y v screw motor Y19a for a Vtime that is'inversely proportional The reluctance device 59v produces atension error signal 75.

tostrip speed signal 72. At the end of this time the pro- One well known cirgr mmer receives a thickness signal 57 that indicates the effect of the last screw operation. ln a typical situation the gauge error would oe smaller than before, and the programmer 53a would order a shorter incremental screw change.

EEG. 2 iilustrates the means for giving screw motor i911 an incremental adiustahly timed operation as an adjustable timer 75 that is connected to receive a strip speed signal 72 and to receive a turn on pulse (produced oy means represented schematically by capacitor 76) when controller 2d is turned oit. When timer is turned on, it runs for a time estabtished by strip speed signal 72, and as it runs it maintains appropriate polarity at its output to keep AND gate closed to prevent operating controller in a digital control, timer 75 may comprise a counter connected to count a predetermined number of pulses in speed signal 2; since each puise corresponds to a predetermined increment of strip length, a preset numher of pulses corresponds to the travel of the corrected region of the strip from stand l to gauge 52.

When the work roll pressure changes at stand i, the strip tension tends to increase or decrease between stands .1L and 2, the tension gauge die orders the motor Zia to speed up or slow down. When the motor 2in is at the limit of its control, it is desirable to prevent the screw motor 19a from further changing the roll pressure since this would change the strip tension. ln the control of this invention the motor rz is limited selectively only when the motor 21a could not match the resulting strip speed change. At all other times the motor ign is free to respond to gauge error signals 57 and to vary the roll pressure over a wide range. When the motor 2in fails t0 maintain the work roll speed in response to tension error signals 6l, the strip tension changes between stands 1 and 2. The tension limit signal 62 then orders the programmer 53a to prevent further changes in roll pressure unti the strip tension returns to the range that the motor lo can control. As FIG. 2 illustrates the controller 63a tension limit signal 62 is connected to allow AND gate 69 to open when the tension is normal and to cause AND gate 59 to close when the tension reaches the predetermined limit.

The rst reducing stand with the screw control that has just been described produces nearly constant gauge in spite of variations in the incoming strip. It is highly desirable to also control the roll pressure at the other reducing stands to improve the uniformity of the strip. The controls for stands 2, 3, 4 and 5 may be simplified by providing only the single thickness gauge 53 near the exit end of the mill, and in some cases the tension limit signal is unnecessary at these stands. The thickness gauge 53 produces a single thickness error order 57 that is Ied into each of the programmers 63?), c, d, e of reducing stands 2 through 5. Programmers 63h, c, d, and e may be similar to the programmer of FIG. 2 except that they do not have the tension input o2. Since the control maintains constant interstand tension, the programmers can operate independently without regard to the effect that the roll pressure changes have on strip speed. As the strip speed changes with changes in the strip or in the roll pressure, the control changes the speeds of the roll motors d Zia, c, d and e correspondingly to prevent changes in interstand tension. isolating the effects of tension changes greatly simplifies applying Wide range controls to each of the stands.

Although the control has been described in terms of a single embodiment, those sliilied in the art will recognize various changes that can be made within the spirit of the invention and the scope of the claims.

What is claimed is:

l. In a control for a hot strip mill having a plurality of reducing stands with screws for positioning the Work rolls of said stands with respect to a rolled strip and with variable speed motors for driving said work rolls, said control comprising means sensing errors the thickness of said rolled strip and ordering error correcting changes in the pressure ot said work rolls, means sensing variations in interstand strip tension and ordering roll drive motor speed changes to maintain constant interstand tension, and means responsive to an uncorrected variation in strip tension to prevent roll pressure changes in a direction to further increase the error in strip tension.

2. In an automatic gauge controi for a hot strip mill having a plurality of reducing stands with means for positioning the work rolls of the stands with respect to a rolled strip and with variable speed motors Jfor driving the work rolls, said control comprising means sensing errors in the thickness of the rolled strip and ordering thickness error correcting changes in the position of said Work rolls, means sensing variations in strip tension and ordering roll drive motor speed changes to maintain constant interstand tension, and means operable when the motor of one of said stands reaches a limit of speed control to prevent roll pressure changes at said one stand in a direction to further increase the error in strip tension.

3. In an automatic gauge control for a hot strip mill having a plurality of reducing stands with means for positioning the Work rolls of the stands with respect to a rolled strip and with variable speed motors for driving said work rolls, said automatic gauge control comprising means sensing errors in the thickness of the strip of rolled material and ordering error correcting changes in the pressure of said work rolls, means sensing variations in strip tension between adjacent stands and ordering changes in the speed of the roll drive motor ot one of said adjacent stands to maintain constant interstand tension, and means operable in response to an uncorrected increase in strip tension between two of said stands to prevent roll pressure changes at one of said two stands in a direction to further increase the strip tension.

References Cited in the file of this patent UNITED STATES PATENTS 2,345,765 Michel Apr. 4, 1944 2,949,799 Walker Aug. 23, 1960 3,036,480 Schwab May 29, 1962 OTHER REFERENCES Control Engineering, September 1956, pages 116417. Iron and Steel Engineer, March 1958, pages 1411-150. Iron and Steel Engineer, July 1958, pages 124-132. 

1. IN A CONTROL FOR A HOT STRIP MILL HAVING A PLURALITY OF REDUCING STANDS WITH SCREWS FOR POSITIONING THE WORK ROLLS OF SAID STANDS WITH RESPECT TO A ROLLED STRIP AND WITH VARIABLE SPEED MOTORS FOR DRIVING SAID WORK ROLLS, SAID CONTROL COMPRISING MEANS SENSING ERRORS IN THE THICKNESS OF SAID ROLLED STRIP AND ORDERING ERROR CORRECTING CHANGES IN THE PRESSURE OF SAID WORK ROLLS, MEANS SENSING VARIATIONS IN INTERSTAND STRIP TENSION AND ORDERING ROLL DRIVE MOTOR SPEED CHANGES TO MAINTAIN CONSTANT INTERSTAND TENSION, AND MEANS RESPONSIVE TO AN UNCORRECTED VARIATION IN STRIP TENSION TO PREVENT ROLL PRESSURE CHANGES IN A DIRECTION TO FURTHER INCREASE THE ERROR IN STRIP TENSION. 